Heat And Mass Transfer Enhancement Of Falling Film Evaporator And Its Applications

Falling film flowing is closely related to the heat and mass transfer process,and has important application value in the industrial field.As the main evaporative equipment in industry,the improvement of heat and mass transfer performances and the development of falling film evaporator are of great practical significance for engineering application,investment and energy saving.By means of experiments and numerical simulation,the paper mainly focuses on the study that using heat transfer enhancement technology to improve the heat and mass transfer performances of falling film evaporator,and developing a new type of falling film evaporator for SO₂ enhanced desorption from basic aluminum sulfate desulphurization regeneration solution.The heat and mass transfer performances of falling film evaporation and sensible heating inside a converging-diverging tube were studied and compared with a built-in spring coil tube and a smooth tube.By analyzing the relationship between the heat transfer coefficient and falling film flow rate,it was found that the converging-diverging tube is suitable for falling film evaporation and sensible heating with a large Reynolds number.When the Reynolds number is larger,the converging-diverging tube has a better heat transfer coefficient than the other two tubes.Within the converging-diverging tube or the built-in spring coil tube,the evaporation heat transfer coefficient is larger than the sensible heating heat transfer coefficient,but both increase with the rise of falling film flow rate.According to the experimental data,the heat transfer correlations of falling film evaporation and sensible heating were obtained.Fluid flow and heat transfer of falling films inside the converging-diverging tube were studied by numerical simulation.By analyzing the falling film thickness distribution,velocity distribution,turbulent intensity distribution,temperature distribution and field synergy,the enhanced heat transfer mechanism was revealed.When the liquid films flow inside the converging-diverging tube,the maximum thickness of the liquid films appears at the largest cross section of the converging-diverging tube,and the minimum thickness is corresponding to the smallest cross section.The average liquid film thickness in the converging segment is less than that in the diverging segment.The radial velocity distribution of the liquid films is semi-parabolic,and the velocity increases with the rise of relative wall distance,reaching the maximum value at the surface of liquid films.Similarly,the velocity distribution in the phase interface layer is semi-parabolic,and the velocity increases with the rise of relative liquid film interface distance,reaching the maximum value at the vapor phase interface.In the converging segment of the converging-diverging tube,the liquid film or vapor flow rate increases gradually,which makes the turbulent intensity of the liquid films or vapor at the two-phase interface increase gradually.In the diverging segment,the liquid film or vapor flow rate decreases gradually,and the turbulent intensity of the liquid films or vapor at the two-phase interface decreases gradually.Compared with the smooth tube,the flow rate of the liquid films inside the converging-diverging tube changes constantly,and the turbulent intensity of the liquid films is higher than that of the smooth tube.The radial temperature gradient increases gradually in the converging segment,and decreases gradually in the diverging segment of the converging-diverging tube.As the overall temperature of the liquid films increases,the radial temperature gradient of liquid films inside the converging-diverging tube and the smooth tube increases gradually.The cosβvalue of flow and temperature gradient field inside the converging-diverging tube is larger than that of the smooth tube,which shows that the flow direction of the liquid films is changed by the converging and diverging rib surface,and the heat transfer enhancement of the flow field and temperature field is coordinated.The heat and mass transfer performances of falling film evaporation and sensible heating in four different sizes of converging-diverging tubes were studied.By analyzing the relationship between the heat transfer coefficient and falling film flow rate,it was found that the key factor affecting the heat transfer performances of falling film evaporation and sensible heating is the rib height.At the same rib height and rib pitch,the longer the converging segment of the converging-diverging tube,the better the heat transfer performance.According to the experimental data,the heat transfer correlations of falling film evaporation and sensible heating for different sizes of converging-diverging tubes were obtained.The effect of converging and diverging rib on the heat transfer performance of falling films was studied by numerical simulation.It was found that the Nusselt number of falling films first decreases and then gradually stabilizes with the rise of rib pitch,first increases and then decreases with the rise of rib height,and first increases and then gradually stabilizes with the rise of converging and diverging ratio.Through field analysis,the mean angle of velocity and temperature gradient field increases with the rise of rib pitch,and decreases with the rise of rib height or converging and diverging ratio.The optimal structural parameter of converging-diverging tube was obtained,and the heat transfer performance of the optimal converging-diverging tube was analyzed and the correlation was obtained.The falling film desorption effect and heat and mass transfer rules of basic aluminum sulfate SO₂-rich solution inside the converging-diverging tube and the smooth tube were studied.It was found as the quantity of SO₂-rich solution increases,the heat transfer coefficient and mass transfer coefficient of falling film evaporation both increase,but the SO₂ desorption efficiency decreases.With the rise of the heating temperature,the three indices all increase.With the rise of the inlet sulfur concentration,the three indices all increase.With the rise of the aluminum concentration,the three indices all gradually decline.With the rise of the basicity,the three indices all gradually decline.The falling film flow rate,heating temperature,sulfur concentration,aluminum concentration and basicity all affect desorption effect to varying degrees,as smaller flow rate,higher heating temperature,higher sulfur concentration,lower aluminum concentration,or lower basicity is more constructive to SO₂ desorption.Comparative analysis showed the heat transfer coefficient and mass transfer coefficient of falling film evaporation,and SO₂ desorption efficiency are larger in the converging-diverging tube than the smooth tube,respectively,indicating the basic aluminum sulfate SO₂-rich solution significantly outperforms in the converging-diverging tube in terms of heat transfer and mass transfer.Moreover,correlations were obtained to predict the heat and mass transfer coefficients and SO₂ desorption efficiency.The basic aluminum sulfate SO₂-rich solution was desorbed via falling liquid film evaporation,which outperformed traditional desorption methods.The effect was better after the use of the enhanced heat transfer and mass transfer technology.